<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    .outer{
      background-color: red;
    }
    .inner1{
      width: 100px;
      height: 100px;
      background-color: orange;
      float: left;
    }
    .inner2{
      width: 100px;
      height: 100px;
      background-color: green;
    }
  </style>
</head>
<body>
<div class="outer">
  <div class="inner1">123</div>
  <div style="clear: both"></div>

  <div class="inner2">456</div>
</div>
</body>
</html>